Isocyanatocarboxylic acids, a process for their preparation and their use in coating compositions as cross-linking agents

A process for the preparation of an isocyanato carboxylic acid containing

• a) 1 to 30% by weight of isocyanate groups attached to (cyclo)aliphatic tertiary carbon atoms and
• b) 0.5 to 500 milliequivalents, per 100 g of isocyanatocarboxylic acid, of carboxyl groups which may be at least partially present in carboxylate salt form,
• which comprises reacting at an NCO/OH equivalent ratio of 1.6:1 to 2:1
• A) a diisocyanate having a molecular weight of 168 to 300, an isocyanate group attached to a primary aliphatic carbon atom and an isocyanate group attached to a tertiary (cyclo)aliphatic carbon atom and
• B) up to 25 isocyanate equivalents percent, based on the equivalents of components A) and B), of one or more diisocyanates other than those set forth in A) which have a molecular weight of 168 to 300 and contain (cyclo)aliphatically bound isocyanate groups with
• C) 2,2-bis-(hydroxymethyl)-alkanoic acids corresponding to the formula ##STR5## wherein R represents hydrogen, a hydroxymethyl group or an alkyl group having 1 to 20 carbon atoms and
• D) optionally one or more organic polyhydroxyl compounds other than those set forth in C).